The occurrence, variety, and proportion of climate change on coral reefs are rising to the level that they are being globally endangered. Estimated increases in sea surface temperature over the following 50 years due to global warming, beat the circumstances under which coral reefs have developed over the past 500 million years. Corals are thermally sensitive, in other words, they can only endure small temperature ranges. However, there is a theory that reefs might suffer modifications rather than vanish completely, with some corals presenting nowadays far superior resilience to coral bleaching than different species. Where bleaching events might be mitigated by their protective mechanism, inducing thermal tolerance of reef-building corals. Based on the finding gap, that some corals are more resilient than others it is fundamental to dig into this feature to better understand their mechanism and make an effort to protect their ecosystem. In order to reduce the rate of coral bleaching and to guide on climate change exploration it is recommended energetic investigation and instigation of international management incorporation on encouraging reef resilience and complement it by solid policy resolutions.
1. Introduction
Coral reefs are acknowledgeable for its enormous diversity and value to the ecosystem on Earth. They have a significant importance since they offer shelter and food to a substantial variety of creatures (Hoegh-Guldberg et al. 2017). Spalding & Brown highlights that reefs uphold more than 3 million species and more than 275 million people that live close by (2015). Besides that, they also play an important role on the planet (Cheal et al. 2017; Hill 2008; Ortiz 2016): protecting coastlines from waves actions, tsunamis, and tropical storms; providing natural resources, such as medicine and food; assisting on nutrient recycling, carbon and nitrogen fixing; generating more than $AU2 billion in tourism in Australia for example (AustralianGovernment 2016).
Little is known about lifespan of a coral. However, it is noticed they are very sensitive ecosystems. Where the most favourable temperature, for an optimum growth, varies between 23 °C and 29 °C and most of them subsist close to the maximum level of thermal temperature (Spalding & Brown 2015). In addition, at this interval of temperature the frequency of bleaching decreases (De’ath, Lough & Fabricius 2009). For instance, in situations when the temperature rises above the normal corals overcome the period of summer standard climate they are adapted to, thereby they reach a state of thermal stress, leading to increasing corals bleaching (Ainsworth et al. 2016).
Notwithstanding the corals importance to the ecosystem and humans, several anthropogenic activities have significantly affected their survival. Over the past two centuries pollution and overfishing from agriculture and land development have been acting as the principal actors of reef ecosystems changes, caused by accelerated reductions in richness of their species. However, at this present moment climate change is the biggest threat to coral reef ecosystems. Changes in water temperature and chemistry caused mainly by humans’ activities and global warming (greenhouse gases) are impacting severely on the coral reefs (Thiagarajan et al. 2013). It is noticeable that coral reefs are decreasing all over the world, reducing their dissemination, abundance and endurance. It is estimated that 30% of the corals are damaged and around 60% may die by 2030 (Hoegh-Guldberg et al. 2017).
Climate change will disturb coral reef ecosystems, throughout sea level increase, alterations to the incidence and strength of tropical storms, and transformed ocean motion configurations. Additionally, carbon dioxide rise on the atmosphere generates it to be absorbed into the ocean in higher rates, reducing calcification taxes in reef-associated organisms and reef-building by varying chemistry of seawater within reducing pH, which is called ocean acidification. Moreover, an increase of temperatures will cause more frequent mass coral bleaching occasions and infectious disease outbreaks. Once these impacts merge the ecosystem purpose will dramatically modify, together with the services and goods coral reef ecosystems deliver to humans everywhere in the world (NOAA 2016).
This present paper purpose is to clarify the complex issue and challenge that coral reefs are facing due to climate change, where the main focus is coral reefs bleaching. Particularly those threats are posed by abrupt changes on water temperature, which is likely to impact negatively the bleaching rate in the near and medium-term future.
2. Literature Review
Lough illustrates the term coral bleaching by being used to designate the coral animal loss of all or some of their photosynthetic pigments and symbiotic algae (2009). It occurs when the corals thermal endurance and their photosynthetic symbionts (zooxanthellae) is surpassed (Hoegh-Guldberg 1999). In other words, the white calcium carbonate skeleton turns out to be visible through the ‘new’ translucent tissue layer, which happens when the symbiotic relationship breaks down and the zooxanthellae living inside corals’ tissue (giving them colour and food) is expelled, consequently bleached corals begin to famish without zooxanthellae and eventually die. Although coral bleaching is being more discussed recently, it is not a new event due to global warming. They are notorious to bleach in reaction to a series of environmental strains, for instance low salinity, pollution and oddly high or low water temperatures.
At one time, coral bleaching incidences could be only witnessed on minor spatial scales in reaction to restricted stresses areas. In 1996, Glynn remarked that the levels of bleaching across coral communities were extremely inconstant and problematic to measure, therefore the comparison among different bleaching events were complicated to evaluate. However, he could affirm the effects of bleaching included high drop or even no skeletal development and reproductive action, as well dropped aptitude to accommodate sediments and fight attack of rival organisms and diseases. Furthermore, when stage of bleaching is severe and persistent could cause part or total colony death, as a result of weakened reef growth, transformation of the reef-building communities, that could lead to non-reef building community types, followed by bio-erosion and finally the vanishing of reef structures. During his researches, he could conclude that the existing evidences implies primary factors accountable for large-scale coral reef bleaching. These elements, also confirmed later on in 1999 by Hoegh-Guldberg, were raised sea temperatures and elevated solar irradiance (particularly ultraviolet wavelengths), where they frequently acted cooperatively. Various authors developed the same conclusion when in 1998 occurred the most widespread and extreme bleaching event on documentation which overlapped with the vigorous El Niño–Southern Oscillation ENSO disturbance on documentation (Kerr 1999). The most important question is whether corals and reef ecosystems would be able to acclimate and adjust to the upcoming warm conditions.
Hughes et al. demonstrated in his studies in 2003, the extent of the global threaten to coral reefs were increasing significantly over the succeeding 50 years, that would outstrip the circumstances in which coral reefs took to flourish over the preceding half-million years. Nevertheless, he developed a conclusion based on available evidences that reefs would adjust rather than vanish completely, since a few species could indicate far superior resilience to coral bleaching and climate change than others. Furthermore, he also concluded the necessity of a deeper research in support of reef management, not only emphasising on routine monitoring and mapping. Yet increasing the scale of tests, selections, and modelling to confirm the scale of effects and main biological procedures over spreading bleaching.
Contradictorily, (Hoegh-Guldberg et al. 2007) during his findings in 2007, claimed that the indications that corals and symbionts could adjust fast to coral bleaching is vague or non-existent. Once reef-building corals have reasonably extensive generation times and little genetic diversity, making the rates of adaptation slower than expected. However, Hoegh-Guldberg and Hughes agreed that corals’ skills to propagate and compete, are forcing reef ecosystems from coral- dominated to algal-dominated states. In this case, the ecosystem may surpass a “tipping point” (Mumby, Hastings & Edwards 2007) and change quickly into an unconventional state with its own inborn resilience and steadiness, making difficult the likelihood of revolving to a coral-dominated state.
Nowadays, the intensification in occurrence of mass coral bleaching occasions is clearly connected to climate change due to the heightened greenhouse effect, affecting entire reefs in numerous negative means (Glynn 1996). Ainsworth et al. puts forward in his studies over the Great Barrier Reef, that bleaching incidents over the previous three decades were alleviated by stimulated thermal resilience of reef-building corals (2016). However, this defensive system is very expected to be misplaced according to the upcoming climate change circumstances. The mechanism consists by a coral state of sub-bleaching stress, before attaining temperatures that trigger bleaching. Yet, findings proved that a slight 0.5°C increase in local temperature would result in loss of this defensive mechanism, that might rise the degree of degeneration of reefs. In addition, Ainsworth analysis uncovers the publicity to lethal pre-stress occurrences changes considerably within reefs, where some possessing an inborn level of ‘defence from’ or ‘alertness form’ to environment, which stimulates coral bleaching, whilst others were able to go through multiple stress exposures in a single event.
There are two ways for a bleached coral reef to recover. First of them is if the surviving retrieve from bleaching by recovering their symbiotic algae plus enduring their growth. The second consists in how effective coral recruitment reloads the reef with youthful corals. However, they are possible to befall if no additional stress is experimented (Craig Reid 2012; Douglas 2003). Additionally, once water temperature returns to usual, bleached corals are able to recover their colour within a week to couple months. Notwithstanding this recover possibility, bleaching has serious and persistent impacts on corals. Further, retrieval of corals’ cover might be gentler than when comparing with other sorts of disturbances (Cantin & Lough 2014).
3. Evaluation
Coral reefs are unique ecosystems, they offer housing to a great quantity of species and marine plants. Further, also provide a vital environment for fish, protect shorelines and are valuable for leisure and tourism. However, coral reef deteriorations will bring frightening consequences for roughly 500 million individuals that rely on coral reefs. Within this, approximately 30 million individuals are totally reliant on coral reefs for their livings or for the land they survive on (NOAA 2016).
The increase of water temperature they have been facing due to climate change, have been causing corals to expel the symbiotic algae that supply nourishment and vibrant colour for their hosts. As mentioned before, this coral bleaching can lead to coral death. Based on corals importance to the humanity and ecosystem and attempting to avoid or minimize extra damages, it is extremely important to deep studies over their behaviour.
Although the connection between raised greenhouse gases, climate change, and corals bleaching were deliberated questioning by many scientists within the previous 20 years, it is now unquestionable. Controversially, the theory of genetic adaptation probably will not work, since a core supposition in the predictions of quick reef degeneration is that genetic alteration in corals and their symbiotic algae will be unsatisfactory to hold pace with climate change (Hoegh-Guldberg et al. 2017). Moreover, thermal stress thresholds of corals have been fairly steady for over 20 years with no measurable change upwards. Researches show that the bleaching and mortality are growing, exposing that stress thresholds are not fluctuating rapidly enough to avoid bleaching. However, adaptation may alleviate the effect of climate change and improve likelihoods of coral reef ecosystem recuperation (Palumbi et al. 2014; Spalding & Brown 2015).
Categorising and defending coral reefs that are more resistant to bleaching and thermal-stress might be analytical on preserving reef ‘s biodiversity and continuing ecological endowment (Donner 2011). Investigation in coral areas susceptible to higher thermal disturbance rates could be critical when identifying the characteristics of resilient coral communities and helping with strategies to enhance coral reefs resilience to climate change. Thus, there is a gap that could save corals survival.
Reef-building corals are bleaching and disappearing at extraordinary rates because of ocean warming. However, corals bleach at extremely dissimilar rates. Corals survival are possible due to association of animals and food provided by symbiont algae. Further, some of those corals have skeletons that disseminate light back to their algae much quicker, rising light accessibility. However, this property makes higher their risk of bleaching (Komyakova, Munday & Jones 2013).
The questions that remains is:
Why some corals are bleaching in such different rates? Would that be due to their biological and/or physical characteristics, making some reefs more resilient or resistant to disorder than others?
My main objective is to identify the skeletal morphological physiognomies that may increase or reduce the risk of bleaching, to better address issues and protect coral reefs.
In order to achieve this goal firstly, I will quantify various skeletal structures, to be able to obtain a high range of data and analyse the outcomes in regard to their light scattering and morphology. Secondly, I will investigate using images and statistics to compare evolutionary similarity of diverse corals which may have developed similar tactics. Thirdly, I will quantify morphology and light scattering at diverse scales in live corals to forecast which of them will be at superior bleaching threat. Finally, after these steps in the future I will monitor the scale of these corals bleach to validate our theory.
4. Conclusion
Climate change is a serious theme that should be more discussed, as it is already substantially impacting coral reef ecosystems. Those effects such as coral bleaching will continue to increase as temperatures increase. As most of the effects of climate change are still to happen, various substantial dramatic hypothesis is possible for several mysterious effects. Presently is known only sufficient to predict a few changes, which includes corals deaths, and a severe change on corals ecosystem.
The gaps found on the recent literature provide an agenda to develop our acquaintance of climate change relationship consequences on corals. Therefore, the question that remnants is how efficaciously some corals and reef ecosystems can better acclimatise and adjust to temperature increase conditions than others. It is very important to distinguish spatial adaptability while aiming management engagements, which target is to moderate coral reef degeneration in the future. Although such practise keeps changing and new study questions can appear throughout unpredicted changes when addressing and fulfilling the research breaches, I hope to attain the understanding required to alleviate the impacts of climate change on corals reef bleaching. Furthermore, though the main focus of this project is on climate change outcomes on coral reef bleaching, the presented question works as a pillar to explore climate changes and, more outstandingly, to classify the most demanding and manageable research guidelines to support focusing on the climate change impacts in the future.